You're asking a rhetorical question, but I'm going to answer it anyway. Please note that this post is for informative purposes, not normative ones.
The thing which is "magical", to use your term, is the family unit comprised of a mother and a father and their children. The children are exposed to both male and female influences, the combination of which is generally recognized as beneficial to their emotional development. The mother and father both have a stake in the upbringing of the children, without attendant conflicts of interest and jealousy. They may also be incentivized towards caring about future generations, building a better world for their children, and a number of other things.
Without that context of the family unit, there is, in fact, significantly less which can be said to be "magic" about the number 2. Notice that "family" occupies a significant portion of the cultural conversation of religious opposition to marriage-related issues, particularly internally-directed conversation (though the likes of 'god hates <x>' is another significant portion and usually louder / easier to find).
Again, this post is informative, not normative. Judgements on the extent to which any of these matters are desirable, and how much they ought to affect the social validity of a marriage of 3, 4, 5, or more, or marriages between 2 people (heterosexual or otherwise) who are not in the business of having children, or alternative ways to achieve any desirable goal mentioned in the above paragraphs, are once again left as exercises to the reader.
Note also that a pragmatic argument against 3+-person marriages exists, and relates to the fact that these are almost always polygynyous marriages and there may be negative social implications to having many unmarried males around, rendering societies more prone to crime, war, and other things. Again, the extent to which this is true and ought to influence social policy? Figure it out yourself.
The thing which is "magical", to use your term, is the family unit comprised of a mother and a father and their children. The children are exposed to both male and female influences, the combination of which is generally recognized as beneficial to their emotional development. The mother and father both have a stake in the upbringing of the children, without attendant conflicts of interest and jealousy. They may also be incentivized towards caring about future generations, building a better world for their children, and a number of other things.
Without that context of the family unit, there is, in fact, significantly less which can be said to be "magic" about the number 2. Notice that "family" occupies a significant portion of the cultural conversation of religious opposition to marriage-related issues, particularly internally-directed conversation (though the likes of 'god hates <x>' is another significant portion and usually louder / easier to find).
Again, this post is informative, not normative. Judgements on the extent to which any of these matters are desirable, and how much they ought to affect the social validity of a marriage of 3, 4, 5, or more, or marriages between 2 people (heterosexual or otherwise) who are not in the business of having children, or alternative ways to achieve any desirable goal mentioned in the above paragraphs, are once again left as exercises to the reader.
Note also that a pragmatic argument against 3+-person marriages exists, and relates to the fact that these are almost always polygynyous marriages and there may be negative social implications to having many unmarried males around, rendering societies more prone to crime, war, and other things. Again, the extent to which this is true and ought to influence social policy? Figure it out yourself.